This invention relates to measuring image characteristics of printed output from a digital printer.
A digital printer receives electronic digital input data in the form of a bitmap (in which there is one bit per pixel), and outputs a printed sheet. The input data are typically received serially in a raster pattern and are used to control the deposit of toner from a drum to a sheet of paper.
Many factors can cause the print quality of a digital printer to deteriorate over time. Traditionally, assessments of this degradation have been done subjectively, as a user tries to match the printer's image quality with his or her subjective perception of ideal output.
Generally, printed sheets are manually inspected to determine quality. The user generates a test page and judges its quality by mentally recalling the appearance of the test page when the printer was new or last serviced. The user can also compare the test page to samples in a "limits" book, which defines the lower bounds for standard printer output. An actual printed image may have defects such as white pinholes in black areas, smears, or blurry edges. Some defects may be acceptable and some may fall below an acceptable quality level. Based on manual inspection, the user rejects printed sheets or makes adjustments to the printer.
A technician performing preventive maintenance may obtain readings of the print density uniformity by using a densitometer, which measures light reflected from a page. These readings only partially correlate with human judgments of print quality. Therefore, unless there is a dramatic shift in quality, or a noticeable defect such as streaking, users have difficulty detecting subtle or gradual changes in quality. With color output, detecting changes is even more difficult since the eye is poor at identifying absolute colors.